Electrostatic pinning in a process for gusseting film web

ABSTRACT

Disclosed is an apparatus and process for gusseting plastic film web having a means for applying an electrostatic charge of negative or positive polarity to the gusset to maintain stable gusset folds prior to storing or processing the gusseted web into a multiplicity of gusseted bags.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 547,416, filed Jul. 3, 1990now U.S. Pat. No. 5,083,999 which is a continuation-in-part applicationof U.S. Ser. No. 272,855 filed Nov. 18, 1988 now abandoned. U.S. Ser.No. 272,855 is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention involves the maintenance of stability of gussetsin thermoplastic bags following the formation of such gussets duringmanufacture of the bag. Gusseted plastic bags are typically manufacturedfrom a substantially continuous sheet of dual-walled plastic filmintegrally connected along at least one continuous edge. The edge of thefilm web opposite the integrally connected edge may be open-sided oropenable along fastening means such as a zipper. Further, the film webmay be tubular in form.

A gusset is formed when an integrally connected film web edge is foldedinward to form folds along that edge as illustrated in Wamsley, U.S.Pat. No. 2,961,930. The film web may then be severed and sealed to formindividual gusseted bags. Upon filling of the gusseted bag, the gussetfolds expand to reveal side or bottom walls of dimension commensuratewith the width of the folds. Gusseted plastic bags have advantages overungusseted plastic bags in that they may be stood upright upon fillingor have rectangular sides.

Numerous apparatus which form gussets in plastic film web are known inthe prior art. Wech, U.S. Pat. No. 3,678,813 and Knuetter, U.S. Pat. No.2,283,069 relate apparatus for gusseting an open-sided film web. Hummelet al., U.S. Pat. No. 4,526,565 and Rochla, U.S. Pat. No. 3,587,907relate an apparatus for gusseting a tubular film web.

A common processing problem encountered is maintainance of stability ofthe gusset after formation thereof. The term stability refers to theextent to which the gusset fold configuration is maintained after thegusset is introduced into the film web. The depth and alignment of thegusset folds with respect to each other and the entire web must besubstantially maintained until the severing and sealing of the film webinto individual bags. Gusset stability after severing and sealing is nota problem since the gusset structure is permanently sealed into theresulting bag.

A common method of enhancing gusset stability is passing the film webthrough mechanical means to press the gusset folds into the film web andmaintain the resulting creases thereafter by providing an adequate levelof process film web tension in the machine direction. These measures maybe inadequate since many types and thicknesses of plastic film web,particularly polyethylene films less than two-thousandths of an inch inthickness, have negligible creasibility.

Another common processing problem encountered with gusseted bags is theformation of leakage points along the side welds of individual bagsafter severing and sealing of the bag film web. The formation of leakagepoints is due to incomplete heat sealing of the edges of the walls ofthe bag including those of the gusset. Air or liquid may leak out of thebag leakage points if the bag is filled with either or both. Aparticular problem leakage area along the side weld is at the top or theinnermost point of the inward fold of the gusset.

Accordingly, there exists a need for a process for gusseting acontinuous, dual-walled thermoplastic film web wherein the stability ofthe guesset is maintained and the incidence of leakage points along theside welds of individual bags formed from the gusseted film web isminimized.

SUMMARY OF THE INVENTION

The invention described herein discloses means for maintaining thestability of gussets in plastic film web after formation thereof in aguesseting apparatus and reducing the incidence of leakage points alongthe side welds of individual bags formed from the gusseted film web.Means for maintaining gusset stability and reducing the incidence ofleakage points include applying an electrostatic charge of negative orpositive polarity to the gusset after formation thereof and prior todisposition of the gusseted film web.

The means for applying an electrostatic charge to the gusset comprise anelectrostatic charging source and an electrically conductive groundedsurface positioned about the gusset of the film web. The electrostaticcharge imparted to the gusset causes the gusset folds or the respectivewalls of each fold to cling to each other at least until the time ofdisposition of the film web.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the present invention and the context within whichthey are set will be better understood upon reviewing the followingspecification together with the several drawings in which the samereference numbers are employed for the same parts in the various viewsand wherein:

FIG. 1 is plan view of an embodiment of an apparatus made in accordancewith the present invention for use with a film web having an open side;

FIG. 2 is a fragmentary cross-sectional view taken along line 2--2 ofthe apparatus of FIG. 3;

FIG. 3 is a plan view of another embodiment of an apparatus made inaccordance with the present invention for use with a film web having azippered edge;

FIG. 4 is a plan view of another embodiment of an apparatus made inaccordance with the present invention for use with a film web havingtubular form; and

FIG. 5 is a cross-sectional view of the apparatus of FIG. 4 taken alongline 5--5.

DETAILED DESCRIPTION OF THE INVENTION

Improved guessting apparatus of the present invention for open-sided andzippered dual-walled webs of plastic film having walls integrallyconnected along at least one continuous edge are referenced generally bythe numberals 17 and 18, respectively, and are illustrated in FIGS. 1and 3, respectively. An improved gusseting apparatus of the presentinvention for tubular, continuous webs of plastic film is referencedgenerally by the numeral 19, and is illustrated in FIG. 4. The threeembodiments described herein are disclosed for purposes of illustration,and are not to be construed as limiting.

The ungusseted film web is referenced generally by the numeral 10, andis so denoted in the specification when referring to ungusseted filmwithout regard to form of web 10, and is readable on reference numerals10a, 10b, and 10c found on the drawings. A web 10 open-sided along theedge opposite the edge of integral connection is specifically referencedby the numeral 10a. The corresponding numerals for web 10 havingzippered or tubular stock are 10b or 10c, respectively. After a gussetor gussets 12 have been introduced therein, film web 10 is generallyrefered to as gusseted film web 11, and is readable in referencenumerals 11a, 11b, and 11c found on the drawings. The correspondingspecific reference numerals of web 11 for open-sided, zippered, andtubular stock are 11a, 11b, and 11c, respectively.

Gusseting apparatus described herein comprise a means of providing aplastic film web 10, a means for formation of a gusset 12, means forexpelling entrapped air from gusseted film web 11, and means forapplying an electrostatic charge to gusset 12 sufficient to cause gussetfolds 13 of gusset 12 to cling together, means for disposing of gussetedfilm web 11, and means for sequentially advancing film web 10 from themeans for providing film web 10 to means for disposing of gusseted filmweb 11.

References to the means, process, or apparatus for forming or formationof gusset 12 differ from references to the process or apparatus forgusseting or references to a gusseting process or apparatus for purposesof the present invention. Apparatus or process for gusseting orgusseting process or apparatus refer to the entire process of gussetingfrom the means of providing film web 10 to the means for disposing ofgusseted film web 11. Means, process, or apparatus for forming orformation of gusset 12 refer only to the physical formation of gusset 12itself.

The means of providing plastic film web 10 for the process of thepresent invention may be fresh-extruded web 10, prior-extruded web 10from a feed roll or other storage receptacle, or extruded web 10 fromanother process stage situated prior to the gusseting apparatus.

The means for formation of gusset 12 in plastic film webs 10a or 10bcomprise two or more thin, closely-spaced, generally parallel andaligned receiving plates 20 and one or more thin protruding plates 26aor 26b protruding between and generally equidistant from and generallyparallel to receiving plates 20. Receiving plates 20 are positionedinside webs 10a and 10b of each respective embodiment contiguous to theintegrally connected edge of webs 10a and 10b to be gusseted. Protrudingplates 26a and 26b are positioned outside webs 10a and 10b contiguous tothe integrally connected edges of webs 10a and 10b to be gusseted. Theextent of protrusion by protruding plates 26 between receiving plates 20is determinative of the depth or width of gusset 12.

The means for formation of gusset 12 film web 10c of tubular dimensioncomprises protruding plate 26c situated outside web 10c contiguous tothe integrally connected edges of web 10c and protruding therein. Apreferred means is illustrated in FIG. 4 and comprises a pair ofgenerally oppositely situated and generally horizontally alignedprotruding plates 26c which form two gussets 12 of preferably equaldepth on opposite sides of web 10c. Plates 26c intercept the planestangential to the integrally connected edges of web 10c. There is enoughair pressure within web 10c at the juncture of web 10c and plates 26c toallow overlap of the integrally connected edge of web 10c over and underplates 26c or portions of plates 26c thereof. It is foreseeable thatplate 26c could be used to form gusset 12 in non-tubular webs such aszippered web 10b if sufficient air pressure is present within.

The preferred material of construction of receiving plates 20 andprotruding plates 26a, 26b, and 26c is metal, though any rigid materialsuch as a rigid plastic or a finished, polished wood may be utilized.

The preferred shaped of protruding plate 26 is thin and circular, thoughany shape such as an ellipse or a rectangle capable of forming gusset 12without damaging the physical integrity of film web 10 may be utilized.Thin and circular protruding plates 26a and 26b are seen in FIGS. 1 and3, and a generally rectangular protruding plate 26c is seen in FIG. 4.The preferred shape of receiving plates 20 is wing-like as in FIGS. 1and 3, though any shape such as a rectangle or a square which is capableof forming gusset 12 without damaging the physical integrity of film web10 may be utilized. Any corners or edges of the protruding plates 26a,26b, and 26c or receiving plates 20 that come into contact with web 10are preferably rounded or smoothed off.

Means for expelling entrapped air from the film web comprise means forsubstantially preventing the passing of entrapped air in gusseted filmweb 11 and means for venting accumulated air. Means for substantiallypreventing the passing of entrapped air in web 11 can comprise a pair ofjuxtapositioned intersecting plates 31 or, preferably, a pair ofjuxtapositioned nip rollers 30 between which web 11 is passed.Intersecting plates 31 are seen in FIGS. 4 and 5 and nip rollers 30a inFIGS. 1 and 3. Nip rollers 30 and intersecting plates 31, being closelyadjacent, press together the walls of web 11, and do not allow thepassing therethrough of a substantial proportion of air entrapped in web11. For film web 11c, intersecting plates 31 provide the additionalfunction of flattening the tubular form to provide more definedintegrally connected edges suitable for formation of gusset 12 thereinthan do nip rollers 30. It is seen that other equivalent means forpressing together the walls of web 11, such as a pair of juxtapositionedflat plates, may be utilized. Accumulated entrapped air trapped behindnip rollers 30 or intersecting plates 31 must be vented to prevent theentrapped air from blowing out gusset 12 after it is formed, as it isformed, or from preventing its formation altogether.

Means for venting accumulated air vary with the configuration of webs 10and 11. For webs 10a and 11a, the open side serves to vent accumulatedair. For webs 10b and 11b, means for venting accumulated air willpreferably comprise bar 22 and closure rollers 32, which are locateddownstream from bar 22. Bar 22 separates rib and groove profiles 15 and16 of zipper 14 as web 10b passes around it. Closure rollers 32 comprisea pair of juxtapositioned rollers which reinterlock rib and grooveprofiles 15 and 16 as zipper 14 passes therethrough. The open side ofweb 11b along zipper 14 serve to vent accumulated air. For all webs 10and 11 but particularly webs 10c and 11c, alternate means of ventingaccumulated air such as an orifice or bleeder valve (not shown) upstreamin the extrusion die may be utilized.

Situated after the means for preventing the passing of entrapped air ingusseted film web 11 is the means for applying an electrostatic chargeto gusset 12 sufficient to cause gusset folds 13 to cling to each other.The means preferably comprise an electrostatic charging source situatedabout and most preferably above the upper surface of gusset 12 of web 11and an electrically-conductive, grounded surface situated about and mostpreferably below the bottom surface of gusset 12 of web 11 in generalopposite alignment with the charging source. The arrangement of theelectrostatic charging source and the electrically conductive groundedsurface with respect to web 11 could, however, be reversed. The groundedsurface is adjacent to film web 11 and, most preferably, contiguousthereto.

A preferred grounded surface comprises an electrically-conductive,grounded roller 42, which substantially traverses the transverse widthof web 11 passing thereover.

A preferred electrostatic charging source comprises an electrostaticcharging head 40 and a power source 43 in electrical connectiontherewith. The preferred positional arrangement for charging head 40 andgrounded roller 42 with respect to each of gusseting apparatus 17, 18,and 19 and film web 10 is illustrated in FIGS. 1, 2, 3, and 4.

Charging head 40 is preferably provided with and is capable of applyingabout 5 to about 30 kilovolts in charge magnitude by power source 43seen in FIGS. 1 and 3, and is situated about 1/4 to about 3 inches abovethe upper surface of the gusset 12. The height and required charge ofcharging head 40 are a direct function of each other. The greater theheight of charging head 40, the greater the required voltage necessaryto stabilize gusset 12. More preferably, charging head 40 is providedwith and is capable of applying about 10 to about 25 kilovolts in chargemagnitude by power source 43, and is situated about 1/2 to about 1 inchabove the upper surface of gusset 12. Most preferably, charging head 40is provided with and is capable of applying about 15 kilovolts in chargemagnitude by power source 43, and is situated about 1 inch above theupper surface of gusset 12. Power source 43 is capable of providing andcharging head 40 is capable of applying electrostatic charge of eitherpositive or negative polarity. Whether positive or negative polarity isdesirable depends upon the polarity characteristics and composition offilm web 10. The preferred polarity for use with a primarilypolyethylene-based film is negative.

A representative suitable power source 43 is the Simco N27C NegativePolarity Static Charging Unit by Simco Co., Inc., and a representativesuitable charging head 40 is the Multipoint Spot Charger 40016obtainable from the same supplier.

The phenomenon by which the electrostatic charge stabilizes gusset 12,i.e. causes gusset folds 13 to cling together or be mutually attractedto one another, is not completely understood, but is believed to resultfrom electron imbalances in gusset folds 13. A negative charging fieldcreated by charging head 40, which is situated above the upper surfaceof gusset 12, is believed to repulse electrons away from that surfacecreating alternating polarities between adjacent contacting surfaces ofthe walls of gusset folds 13. The opposite polarities between contactingsurfaces causes the walls of folds 13 and folds 13 themselves to clingtogether. Likewise, a positively charged charging head 40 may createalternating polarities reverse that of a negatively charged charginghead 40.

The means for applying an electrostatic charge of sufficient magnitudeto the gusset to cause the gusset folds to cling to each other alsoincludes those devices which utilize two electrostatic charging sources(not shown) of opposite polarity situated oppositely about the gusset.Further, the means includes those devices which utilize direct current,alternating current, or a combination of both as seen in Levy,3,892,614, the teachings of which are herein incorporated by reference.

The mutual attraction or cling of gusset folds 13 from the applicationof an electrostatic charge thereto has been observed to last as long asa week. Such attraction may be necessary for only a few minutes ifgusseted film web 11 is being severed and sealed into individual bagssoon after application of the charge. This sustained attraction can beuseful, however, in that it allows bifurcation of the steps of forminggusset 12 in film web 10 and severing and sealing of web 11 into bags.Web 11 can be wound on rolls or other storage receptacles and severedand sealed later until substantial charge dissipation has taken place.This residual charge imbalance is believed to be due to thesemi-conductive nature of many types of film compositions for plasticbags. The films are generally conductive to static charging from astrong external source, but exhibit only gradual dissipation of acquiredcharge.

Situated after the means for applying an electrostatic charge to gusset12 is the means for disposing of gusseted film web 11. The means fordisposing of web 11 will usually comprise either means for severing andsealing of web 11 into individual bags (not shown) or means forwindingup of web 11 on a takeup roll or other storage receptacle (not shown)for subsequent storage, transportation, processing, and eventualsevering and sealing into individual bags. Representative means forsevering and sealing of web 11 into individual bags are seen in thesealing apparatuses of Tumminia, U.S. Pat. No. 4,396,449 and Carland,U.S. Pat. No. 2,711,779, both of which are incorporated herein byreference.

The application of an electrostatic charge to gusseted film web 11results in an additional advantage in a lower incidence of formation ofleakage points along the side welds of individual bags formed from thesevering and sealing of film web 11. Though not wishing to be bound byany particular theory, the lower incidence of leakage points is believedto be due to a higher degree of cling and a tighter and closerpositioning of the walls of gusseted film web 11 including gusset 12itself due to induced static charges. The closer positioning of thewalls of gusseted film web 11 result in side welds with a lowerincidence of leakage points after severing and sealing by a heated wiresuch as seen in U.S. Pat. No. 4,396,449 and U.S. Pat. No. 2,711,779.

Means for advancing web 10 from means for providing web 10 to means fordisposing of web 11, or, in other words, from start to finish, refers toany mechanical or electromechanical process or urging mechanism foraccomplishing the same. Examples of such means include mechanically orelectrically driven takeup mandrels or a coordinated drive rollersystem. Webs 10 and 11 may be either sequentially or continuouslyadvanced.

While the preferred embodiments of the invention have been shown withregard to specific details in gusseting processes, it will beappreciated that depending upon the gusseting process and themanufacturer's desires, the invention may be modified by various changeswhile still being fairly within the scope of the general teachings andprinciples hereof.

What is claimed is:
 1. Apparatus for gusseting a continuous, dual-walledplastic film web having walls integrally connected along at least onecontinuous edge, comprising means for providing the web, means forforming a gusset having gusset folds extending continuously along atleast one integrally connected edge of the web, means for applying anelectrostatic charge of sufficient magnitude to the gusset to cause thegusset folds to cling to each other, and means for disposing of the filmweb.
 2. The apparatus as recited in claim 1, wherein the means forforming a gusset comprise two or more closely-spaced receiving platesand one or more protruding plates, the receiving plates being generallyparallel and aligned, the protruding plates protruding between and beinggenerally equidistant from and generally parallel to the receivingplates, and the receiving plates being positioned to form a gusset inthe web at the integrally formed edge when the web is drawn over thereceiving plates and around the protruding plate.
 3. The apparatus asrecited in claim 1, wherein the means for forming a gusset comprise aprotruding plate, the protruding plate being positioned to form a gussetalong the integrally connected edge when the web is drawn along and pastthe protruding plate.
 4. The apparatus as recited in claim 1, whereinthe means for applying an electrostatic charge to the gusset comprise anelectrostatic charging source and an electrically-conductive, groundedsurface, the charging source and the grounded surface being positionedto apply an electrostatic charge to the gusset passing therebetween, thecharging source and the grounded surface being generally oppositelysituated about the gusset passing therebetween, and the grounded surfacebeing adjacent to the gusset passing therebetween.
 5. The apparatusaccording to claim 4, wherein the electrostatic charging sourcecomprises an electrostatic power source and an electrostatic charginghead, the head being in electrical connection with the power source. 6.The apparatus according to claim 5, wherein the charging head issituated about 1/4 to 3 inches from the plane along which the gussetwill be drawn, and the power source being capable of providing and thecharging head being capable of applying from about 5 to 30 kilovolts ofcharge.
 7. The apparatus according to claim 6, wherein the charging headis situated about 1/2 to 1 inch from the plane along which the gussetwill be drawn, the power source being capable of providing and thecharging head being capable of applying about 10 to 25 kilovolts ofcharge.
 8. The apparatus according to claim 7, wherein the charging headis situated about 1 inch from the plane along which the gusset will bedrawn, the power source being capable of providing and charging headbeing capable of applying from about 15 kilovolts of charge.
 9. Theapparatus according to claim 4, wherein the electrically conductive,grounded surface comprises an electrically conductive, grounded roller.